1. Field of the Invention
The present invention relates to impedance converter circuits to be included in antenna devices and the like, and more particularly to an impedance converter circuit that achieves matching in a wider frequency band range and a communication terminal device including this impedance converter circuit.
2. Description of the Related Art
In order to become compatible with downsized radio communication devices such as cellular phone terminals, a single radiation element is shared with a plurality of communications systems in many cases. In a case that a single radiation element is shared with communication systems of a low band (for example, 800 MHz) and a high band (for example, 2 GHz band), a base resonant mode and a higher-order resonant mode of this single radiation element are used. However, the impedance of the radiation element varies depending on the frequency. Thus, there is an issue in that, when a matching circuit is made to match with one of the frequency bands, the matching circuit may not achieve matching at another frequency.
To resolve the foregoing issue, an impedance converter circuit in which a transformer circuit is used for a matching circuit is proposed as described in Japanese Patent No. 4761009.
Furthermore, as described in International Publication No. WO 2012/153691, another impedance converter circuit is also proposed. Here, the impedance converter circuit is provided with a bypass capacitor so that a low band (800 MHz band) signal mainly passes through a matching circuit and a high band (2 GHz band) signal mainly passes through the bypass capacitor.
For example, in small mobile terminals such as smartphones, antenna impedances at the 800 MHz band and the 2 GHz band are usually lower than the impedance of RFIC antenna port. Thus, the impedance converter circuit described in Japanese Patent No. 4761009 is effective. However, for example, assume a case that a condition is applied so as to achieve matching at the high band (2 GHz band) without adding an impedance converter circuit. In such case, the matching may be achieved at the low band by adding an impedance converter circuit, but the matching may not be achieved at the high band.
FIG. 13A and FIG. 13B depict an example in which no impedance converter circuit is provided. Here, a matching state is achieved at the high band, but a mismatching state occurs at the low band. FIG. 13A is a frequency characteristic diagram of return loss RL and insertion loss IL when an antenna is viewed from a power feed port, and FIG. 13B is a diagram in which impedances are represented on a Smith chart when the antenna is viewed from the power feed port.
In FIG. 13A and FIG. 13B, frequencies at respective markers are as follows (the same applies to FIGS. 14A and 14B and FIGS. 16A and 16B):
m1, m7, m11: 700 MHz
m2, m8, m12: 960 MHz
m3, m9, m13: 1.71 GHz
m4, m10, m14: 2.7 GHz
The foregoing range of 700 MHz to 960 MHz is the low band, and the foregoing range of 1.71 GHz to 2.7 GHz is the high band.
On the other hand, FIG. 14A and FIG. 14B are diagrams depicting a state that is changed after inserting an impedance converter circuit between a power feed circuit and an antenna. FIG. 14A is a frequency characteristic diagram of return loss RL and insertion loss IL when an impedance converter circuit side is viewed from a power feed port, and FIG. 14B is a diagram in which impedances are represented on a Smith chart when the impedance converter circuit side is viewed from the power feed port.
As depicted in FIG. 14B, impedance matching may be obtained at the low band represented with m1-m2 because of an effect of the impedance converter circuit. In the high band represented with m3-m4, however, as depicted with a circling dashed-dotted line, a circle is reduced in size and shifted to a higher impedance side.
As described above, an impedance-converting transformer circuit converts the impedance over a wide band range. Thus, it is difficult to make the transformer effective only at a specific frequency band. Accordingly, shifting of the matching by the impedance converter circuit becomes an issue.
On the other hand, in the impedance converter circuit described in International Publication No. WO 2012/153691, using the capacitor for bypassing becomes difficult in a case that an inductance component of the transformer circuit is small.
FIG. 15 depicts an example in which an impedance converter circuit including a transformer T1 and a bypass capacitor Cp is inserted between a power feed circuit 11 and an antenna element 12. FIG. 16A is a frequency characteristic diagram of return loss RL and insertion loss IL when an impedance converter circuit side is viewed from the power feed circuit 11, and FIG. 16B is a diagram in which impedances are represented on a Smith chart when the impedance converter circuit side is viewed from the power feed circuit 11. The frequencies of respective markers are the same as those in the above. Here, the capacitance of the bypass capacitor Cp is 15 pF. To reduce the insertion loss of the transformer T1, a reduction of the inductance of a primary coil L1 at the transformer T1 is effective. However, this also reduces the inductance of a secondary coil L2. Consequently, a high band signal is shunted at the secondary coil L2, thereby reducing the passing amount through the bypass capacitor Cp. For example, in a case that the inductance of the secondary coil L2 is equal to 5 nH or less, matching is not achieved at the high band as depicted in FIG. 16A and FIG. 16B.